1. Field of the Invention
This invention relates to optical fibers and, more particularly, to multi-wavelength, multimode silica optical fibers.
2. Discussion of the Related Art
The manufacture of multi-transverse-mode optical fiber (simply multimode fiber hereinafter) has reached a very sophisticated level of development. Yet, in some cases, multimode fiber specifications are so stringent that it is difficult to develop materials and processes capable of satisfying requisite specifications. For example, the properties of many high bandwidth fibers, particularly their dispersion properties, are extremely sensitive to variations in the diameter of the fiber core and its refractive index profile. In fact, calculations for some commercially-available multimode fibers have shown that as little as a 1% variation in core profile induces up to a 10% variation in dispersion. Due to this dispersion effect, specifications for such fiber generally are applicable to only one center wavelength of operation. With that limitation, it is sometimes difficult to achieve adequate bandwidth to meet customer specifications. This bandwidth limitation could be alleviated if the multimode fiber were capable of transmitting an additional center wavelength without introducing significant dispersion; that is, if the multimode fiber were also a multi-wavelength fiber. In this regard, the transmission wavelengths should be sufficiently separate from one another in the frequency domain that they do not significantly overlap, which in turn means that they are separated by more than the linewidth of the center wavelengths.
Early investigators of multimode fiber designs recognized that a parabolic refractive index profile in the core substantially reduced the intermodal dispersion in the fiber. However, they assumed that this parabolic profile would be optimum and that it would be the same for all transmission wavelengths and fiber compositions. This approach did not take into account the variation in refractive index dispersion in different material compositions from which the fibers were constructed. Around 1975, Keck and Olshansky recognized that the variation in dispersive properties of core and cladding materials in multimode fiber did affect the optimum profile shape for any wavelength of operation. They described the now standard representation used to calculate the optimum refractive index profile shape in optical fiber in U.S. Pat. No. 3,904,268 issued on Sep. 9, 1975, which is incorporated herein by reference. In this representation the refractive index nc(r) of the core at any radius, r, less than the core radius, α, is given bync(r)=nc1[1−2Δ(r/a)α]1/2  (1)whereαopt=2+y−[Δ(4+y)(3+y)/(5+2y)],  (2)Δ=(nc12−nc22)/2nc12,  (3)y=−(2nc1/N1)(λdΔ/dλ)/Δ,  (4)andN1=nc1−(λd nc1/dλ).  (5)The quantities nc1 and nc2 are the refractive indices of the core at r=0 and r=a, respectively, and λ is the operating wavelength of the system incorporating the optical fiber as a transmission medium. Prior to recognition of the impact of refractive index dispersion through the y-parameter in equation (4), αopt, the optimum profile shape parameter, was expected to be equal to two for all fiber transmission wavelengths.
Following the work of Keck and Olshansky, however, it was recognized that the optimum profile shape varied significantly as a function of transmission wavelength based on the significant variation in dispersion of the component glasses of the multimode optical fiber. Prior art workers suggested several methods to reduce the y-parameter and thereby to obtain a multimode fiber in which the profile shape was more nearly independent of wavelength. In other words, their objective was to design a multimode compositional structure where dαopt/dλ=0. For example, in U.S. Pat. No. 4,105,283, which issued on Aug. 8, 1978 and is incorporated herein by reference, D. C. Gloge et al. theoretically outlined a process for modifying the y-parameter by observing the necessary relationships between the dispersions of fiber core and cladding materials. However, they did not actually identify specific materials that had those dispersive characteristics and that could be used to achieve the theoretical profile shapes. In U.S. Pat. No. 4,025,156, which issued on May 24, 1977 and is also incorporated herein by reference, Gloge et al. did describe a specific compositional system, the GeO2—B2O3—SiO2 glass system, that exhibited the property of dαopt/dλ˜0 for a broad range of wavelengths for multimode fiber. Their example was a multimode fiber that had an NA sufficiently below 0.2 that the fiber would fail to satisfy the current standard for multimode fiber. In addition, J. W. Fleming discovered the same concept in the P2O5—B2O3—SiO2 glass system. (See, U.S. Pat. No. 4,033,667, which issued on Jul. 5, 1977 and is incorporated herein by reference.) He found that this glass system has a 500 nm wavelength range for which dαopt/dλ˜0. In this system the fiber NA can easily be made to exceed 0.2.
While the importance of wavelength independence of optimum profile shape was well known in this period of time, the fiber core compositions that provided the appropriate dispersion were very few. Moreover, the few suitable compositions were found to exhibit other problems, such as environmental sensitivity or manufacturing difficulty. As a result, the B2O3—SiO2 glass system did not become the standard for multimode fiber production; the GeO2—SiO2 system did. Cores that depend on GeO2 for refractive index profiling, however, do not exhibit the optimum profile shape for wavelength independence and can be optimized for only one transmission wavelength. But glasses in the GeO2—SiO2 system are easier to fabricate into optical fiber cores using existing vapor phase methods, such as MCVD, PCVD, and OVD. To date wavelength independence of the optimum profile shape has not been commercially achieved in the 0.78 to 1.55 μm range (nor in the narrower 0.85 to 1.3 μm range) by any manufacturer of multimode fiber.
Thus, a need remains in the art for a multimode fiber that has an essentially optimum core profile shape that is essentially independent of wavelength over a predetermined range of operating wavelengths of the fiber.